Satellite systems are used for many reasons, including observation, communication, navigation, weather monitoring and research. Satellites and their orbits can vary widely, depending on their function. One common classification system for satellites is based on their orbit, for example, low earth orbit (LEO), polar orbit or geostationary orbit.
A LEO satellite system is commonly defined as having an orbit between 160 kilometers and 2000 kilometers above the Earth's surface. It has many purposes, particularly for communication systems, since a less powerful amplifier is required for transmission than for satellites with higher orbits. LEO satellites travel at a high velocity in order to maintain their orbit, and typically make one complete revolution around the Earth in about 90 minutes. Since they are not geostationary, LEO satellites are used in a network, or constellation, of several linked satellites to provide continuous transmission coverage. Receivers are positioned at various places around the Earth and communicate with any given LEO satellite only when it is within range. By the same token, the satellite communicates with any given receiver for only a portion of its orbit.
Satellite systems are costly, operate in widely varying link conditions, and generally have long transmission delays. Transmissions between satellites and ground-based receivers can also be heavily impacted by the background noise. These transmissions are characterized by a signal-to-noise ratio (SNR) which is the ratio of the signal power to the noise power. The channel capacity of a LEO satellite communication, CLEO, is typically given byCLEO=BW×log2(1+SNR)
where BW denotes available bandwidth for communication and SNR denotes Signal-to-Noise ratio. This formula is known as the Shannon limit or Shannon capacity and is the theoretical maximum information transfer rate of the channel, for a particular noise level.
Initially, prior art satellite communication systems used a fixed rate transmission. This required no feedback but was highly sub-optimal when used in a system with widely varying received SNR. An improvement on fixed transmission is adaptive coded modulation (ACM) that measures SNR in real-time and provides feedback to govern the ACM transmission rate.
It is well-known that adaptive coded modulation (ACM) will maximize the throughput of a channel based on the current channel conditions. Traditional satellite communication systems (i.e. either LEO or GEO) require a feedback channel (i.e. from the ground receiver (Rx) to satellite transmitter (Tx)) so they can adaptively change coding rate and modulation format at the transmitter in response to conditions at the receiver. A conventional ACM method relies on obtaining current channel state information (CSI) from the feedback channel. Thus, it is essentially a “closed-loop” communication system as depicted in FIG. 1. As shown in FIG. 1, an ACM module 102 in a LEO satellite provides ACM data to transmitter 104 which is sent over a communication channel 106 to receiver 108. Receiver 108 provides the received data to ACM module 110 which demodulates the data and provides feedback to ACM module 102 about received SNR using feedback channel 112. However, long transmission delays in a satellite system can make near real-time feedback difficult to accomplish.
Thus, a need exists to provide ACM in a LEO satellite system without requiring real-time feedback of received SNR.